Environmental Engineering Reference
In-Depth Information
THE EFFECT OF EARTH'S ROTATION
The rotation of Earth causes the winds to be deflected from the simple pattern just
identified. The deflection is towards the right in the northern hemisphere and towards the
left in the southern hemisphere. Instead of a direct meridional flow, the Coriolis force
produces a surface flow similar to that shown in Figure 6.1
This is not the only effect of Earth's rotation. Air moving towards the poles from the
tropics forms a series of irregular eddies, embedded within the generally westerly flow.
These can be seen on the satellite photographs as spiralling cloud patterns, similar to the
patterns we can see in a turbulent river (Plate 6.1).
Again we can understand the cause of these eddies with the help of a simple
experiment. A pan of water is heated at the rim and cooled at the centre. If the pan is
slowly rotated it is seen that a simple thermal circulation is produced. If the rate of
rotation is increased, however, the flow suddenly becomes unstable. New patterns form
like those we see in the atmosphere of the temperate latitudes - eddies and waves. It
seems that rapid rotation, like that of Earth, sets up forces which disturb the simple
circulation of the atmosphere, particularly near the axis of rotation (i.e. in higher
latitudes). These forces destroy the simple pattern and produce more complex circulation
(Plate 6.2).
THE EFFECT OF SURFACE CONFIGURATION
Even now our picture of atmospheric circulation is far from complete. Earth's surface is
not uniform, and the variations in its surface form cause ever more disruption of the
pattern of circulation. Friction affects the winds, reducing the effect of the Coriolis force,
and, locally, it deflects the surface flow of air to produce highly complicated systems of
movement. Temperature differences produced by different types of surface, such as land
and sea, also have an impact.
It is difficult to model the effects of surface configuration, but a general indication of
its influence can be obtained by comparing the northern and southern hemispheres. In the
northern hemisphere there are extensive and irregular land masses. Much of the southern
hemisphere, by contrast, is ocean, except for the high ice plateau of Antarctica, where
very low temperatures are experienced. As we might expect, the pattern is much simpler
in the southern hemisphere. A strong westerly flow of cool polar air occurs even in the
southern summer. Conversely, in the northern hemisphere the flow is weaker and more
irregular, with major meridional air movements and seasonal variations. The temperature
differences between pole and equator are less marked (about 30° C compared with 60° C
in the southern hemisphere) and so the driving force of the winds - the pressure gradient
- is reduced.
ENERGY TRANSFER IN THE ATMOSPHERE
The pattern of energy transfer in the atmosphere is complex, and we can consider here
only some of the general components of the pattern. As a starting point, let us look at the
simplified model of what happens in the tropics (Figure 6.12).
